In order to meet continual needs to increase density for CMOS electronics, the size of devices is constantly decreasing. However, these devices have more and more problems of energy consumption, particularly in static mode, due to an increased leakage current linked to their reduction in size.
Non-volatile memories, which conserve their data in the absence of power supply, are very interesting in order to reduce energy consumption. MRAM (magnetic random access memory), which are both non-volatile and capable of assuring rapid reading and writing times, are thus particularly interesting. This type of memory is based on magnetic tunnel junctions, formed by two ferromagnetic layers separated by an insulating oxide, generally magnesium oxide MgO. The resistance of the device varies typically by a factor of 2 to 3 according to whether the magnetisation of the two ferromagnetic layers is parallel or antiparallel, thus providing a magnetic “0” or “1”. In an electronic circuit, non-volatility makes it possible to reduce energy consumption by switching off the temporarily inactive parts of the circuit and thus by eliminating leakage current in these parts. New strategies for triggering the power supply are thus rendered possible. The concept of electronic computing in a circuit which is normally off with instantaneous switch on (known as “Normally-Off/Instant-On computing”) has even been introduced to describe this approach (“Challenges toward gigabit-scale spin-transfer torque random access memory and beyond for normally off, green information technology infrastructure”, Takayuki Kavvahara, J. Appl. Phys. 109, 07D325, 2011).
However, the writing mechanisms of these devices always necessitate a relatively high amount of energy. The least favourable configuration is that of writing with a magnetic field induced by a circulation of current in a conducting line situated in the immediate proximity of the magnetic tunnel junction (generally above or below). In this case, writing requires currents of the order of several mA for around 10 ns. The energy associated with the writing of an event is then several tens of pJ. A memory cell with magnetic field writing 100 is schematically represented in FIG. 1a. The memory cell with magnetic field writing 100 is composed of a first magnetic layer 101 and a second magnetic layer 102, separated by a layer of oxide 103 forming a tunnel barrier. The magnetisation of the first magnetic layer 101, called reference layer, is set in a fixed direction. The magnetisation of the second magnetic layer 102, called storage layer, may be oriented in different directions with respect to the magnetisation of the reference layer 101. A first current line 104 and a second current line 105, situated in the vicinity of the magnetic tunnel junction and generating respectively a first magnetic field Hx and a second magnetic field Hy, make it possible to modify the magnetisation of the storage layer 102.
The use of STT (spin transfer torque) for writing in a magnetic tunnel junction provides a better configuration, in particular when the density is high, that is to say when the memory is of small size (typically less than 50 nm diameter). In fact, in this case, switching is determined by current density. Thus, the smaller the magnetic tunnel junction, the smaller the current required for writing, since the current is equal to the current density multiplied by the area of the magnetic tunnel junction.
At present, magnetic tunnel junctions which have magnetisation perpendicular to the plane of the layers (“perpendicular-to-plane magnetisation”) are attracting a lot of attention because they require, for a given writing, a much smaller current density than that required by a magnetic tunnel junction having a magnetisation parallel to the plane of the layers (“in-plane magnetisation”). During spin transfer writing, which will be designated “STT writing” in the remainder of this document, the writing energy is of the order of several pJ, which remains well above the energy typically required to switch on and switch off a CMOS transistor and which is of the order of fJ. This writing energy is mainly associated with dissipation by Joule effect in the magnetic tunnel junction and in the conducting wires connected to the electrodes of the magnetic tunnel junction. A magnetic memory cell with spin transfer writing 110 with magnetisation in the plane of the layers, known as IP (“in plane”) is represented schematically in FIG. 1b. The magnetic memory cell with spin transfer writing 110 is composed of a first magnetic layer 111 and a second magnetic layer 112, separated by a layer of oxide 113 forming a tunnel barrier. The magnetisation of the first magnetic layer 111, called reference layer, is set in a fixed direction in the plane of the layers. The magnetisation of the magnetic layer 112, called storage layer, may be oriented in different directions of the plane of the layers. A first electrode 114 is placed in contact with the storage layer 112. A second electrode 115 is placed in contact with the reference layer 111. A sufficiently important spin polarised current applied through the magnetic tunnel junction by means of first and second electrodes 114 and 115 makes it possible to exert on the magnetisation of the storage layer 112 a torque capable of modifying it.
A magnetic memory cell with spin transfer writing 120 with so-called OP (“out of plane”) magnetisation of the layers is schematically represented in FIG. 1c. The magnetic memory cell 120 differs from the magnetic memory cell 110 in that:                the magnetisation of the reference layer 121 is set in a fixed direction out of plane of the layers;        the magnetisation of the storage layer 122 may be oriented in different directions out of plane of the layers.        
Nevertheless, a drawback of STT writing is linked to the reliability of the tunnel barrier. In fact, the memory is crossed by a relatively high current density, of the order of MA/cm2, at each writing which induces a risk of dielectric breakdown. Thermally assisted switching, which consists in assisting the switching of magnetisation by temporary heating of the magnetic tunnel junction, makes it possible to facilitate switching at the moment of writing while assuring very good stability of the magnetisation outside of writing regime.
A memory cell with thermally assisted magnetic field writing 100′ is schematically represented in FIG. 1d. The memory cell with thermally assisted magnetic field writing 100′ differs from the memory cell with magnetic field writing 100 in that a current is applied through the magnetic tunnel junction, prior to the switching of magnetisation of the storage layer 102, so as to reduce substantially the magnetic field to apply to the junction to carry out the switching of magnetisation. Nevertheless, electric field writing always leads to high energy consumption.
A magnetic memory cell with thermally assisted spin transfer writing with magnetisation “in the plane” 110′ is schematically represented in FIG. 1e. The memory cell with thermally assisted spin transfer writing 110′ differs from the magnetic memory cell with spin transfer writing 110 in that the current flowing through the magnetic tunnel junction is used both to heat the storage layer of the cell and to exert the STT torque enabling the switching of the magnetisation of the storage layer 112, so as to reduce substantially the current to apply to the junction to carry out the switching of magnetisation.
A magnetic memory cell with thermally assisted spin transfer writing with out of plane magnetisation 120′ is represented schematically in FIG. 1f. The memory cell with thermally assisted spin transfer writing differs from the magnetic memory cell with spin transfer writing 120 in that the current flowing through the magnetic tunnel junction is used both to heat the storage layer of the cell and to exert the STT torque enabling the switching of the magnetisation of the storage layer 122, so as to reduce substantially the current to apply to the junction to carry out the switching of magnetisation. However, these two latter devices still pose problems of reliability.
A new approach for changing the resistance of a magnetic tunnel junction with perpendicular magnetisation, that is to say to change the direction of magnetisation, consists in using an electric field with extremely weak currents crossing the device. Rapid switching with low energy consumption has been obtained with this type of device (“Induction of coherent magnetization switching in a few atomic layers of FeCo using voltage pulse”, Yoichi Shiota et al. Nature materials, 11, 39, 2012).
The possibility of changing the magnetisation of a thin magnetic metallic film with an electric field is due to a change in the magnetic anisotropy of the film thanks to the electric field applied. Given that in metals the electric field is screened over a very short distance, called “Fermi distance” and which is 0.2 nm in metals commonly used for the electrodes of magnetic tunnel junctions, the influence of the electric field on the anisotropy is uniquely interfacial. In the case where the thin ferromagnetic film is in contact with an insulating layer, an electric field may be applied through the insulating layer and in the magnetic metallic layer over the Fermi screening distance. This field can locally change the state density along the interface between the magnetic layer and the insulating layer, which can in turn modify the surface anisotropy which exists at this interface (“First-principles investigation of the very large perpendicular magnetic anisotropy at Fe/MgO and Co/MgO interfaces”, Yang et al., Physical Review B 84, 054401, 2011). This is the case in particular at interfaces of buffer layer/CoFeB/MgO or buffer layer/FeCo/MgO type, which are of very great practical importance in magnetic tunnel junctions. This modification leads to a change in the effective anisotropy Keff, which leads to an effective anisotropy perpendicular to the plane or in the plane, according to the equation:
      K    eff    =                    -                  1          2                    ⁢              μ        0            ⁢              M        s        2              +          K      V        +                                        K                          s              ⁢                                                          ⁢              1                                ⁡                      (            V            )                          +                  K                      s            ⁢                                                  ⁢            2                                      t        F            
In this equation, Ks2 is the surface anisotropy at the buffer layer/magnetic metal interface; Ks1 is the surface anisotropy at the magnetic metal/oxide interface; −½μ0Ms2+KV is the volume anisotropy which includes in its first term the shape anisotropy and in its second term the magnetocrystalline anisotropy; tF is the thickness of the ferromagnetic film.
The effective anisotropy Keff is the sum of the contributions of the surface anisotropies related to the volume by dividing by the thickness of the ferromagnetic film, and of the volume anisotropies.
FIG. 2 is a graphic representation of the evolution of the effective anisotropy Keff multiplied by the thickness tF of the ferromagnetic film, as a function of the thickness tF of the ferromagnetic film. This graphic representation is a straight line of which the slope is proportional to the contribution of the volume anisotropy, that is to say to the shape anisotropy −½μ0Ms2, plus the magnetocrystalline anisotropy Kv, the magnetocrystalline anisotropy nevertheless being generally negligible in the devices implemented.
By convention, in magnetism, when an effective anisotropy is positive, the magnetisation is spontaneously oriented out of plane. On the contrary, when an anisotropy is negative, the magnetisation is spontaneously oriented in plane. Hence, throughout the present application, a switching of anisotropy will signify, by a commonly employed misuse of language, a change of sign of the anisotropy which leads to a spontaneous switching of the orientation of the magnetisation of the layer considered.
The contribution of the shape anisotropy is always a negative value because it contributes systematically to an in plane orientation of the magnetisation.
The intersection of the straight line with the straight line of equation tF=0 gives the contribution of the surface anisotropy Ks1+Ks2, which is here positive (see FIG. 2) and thus contributes to an out of plane orientation of the magnetisation.
When an electric field is applied, only the surface anisotropy is capable of being modified on account of the screening of the field in the magnetic metallic electrode, which thus leads to changes in the curves. These changes are explained in relation with FIG. 3a. FIG. 3a shows:                a first graphic representation 31, when no polarisation voltage is applied (V=0), of the effective anisotropy Keff multiplied by the thickness tF of the ferromagnetic film, as a function of the thickness tF of the ferromagnetic film;        a second graphic representation 32, when a non-zero polarisation voltage V=Vmax is applied to the terminals of the device, of the effective anisotropy Keff multiplied by the thickness tF of the ferromagnetic film, as a function of the thickness tF of the ferromagnetic film.        
A translation along the y-axis of the second graphic representation 32 compared to the first graphic representation 31 is observed.
If the thickness of the ferromagnetic film is correctly chosen, as illustrated in FIG. 3a for a thickness tc of the ferromagnetic film, the variation in surface anisotropy may lead to a change in sign of all the effective anisotropy. In the case illustrated in FIG. 3a, there is thus an anisotropy perpendicular to the plane when no polarisation voltage is applied, and an anisotropy in the plane when an electric field is applied through the magnetic metal/oxide interface thanks to a polarisation voltage V=Vmax applied through the oxide layer.
The opposite case is also possible and is illustrated in FIG. 3b. FIG. 3b shows:                a third graphic representation 33, when no polarisation voltage is applied (V=0), of the effective anisotropy Keff multiplied by the thickness tF of the ferromagnetic film, as a function of the thickness tF of the ferromagnetic film;        a fourth graphic representation 34, when a non-zero polarisation voltage V=Vmax is applied, of the effective anisotropy Keff multiplied by the thickness tF of the ferromagnetic film, as a function of the thickness tF of the ferromagnetic film.        
Starting from an anisotropy in the plane when no polarisation voltage is applied (V=0), it is possible to obtain an anisotropy perpendicular to the plane by applying a voltage V=Vmax. This second configuration is obtained by changing the effective anisotropy in order that it is negative when no polarisation voltage is applied (V=0) and by applying a polarisation voltage of inverse sign in order to obtain a positive effective anisotropy at V=Vmax.
It is thus possible to control the switching of the effective anisotropy, from a direction perpendicular to the plane to a direction in the plane and vice versa, as a function of the application or not of a polarisation voltage. Taking as an example the case of an initial magnetisation along a direction in the plane, and an effective anisotropy:                along a direction in the plane when no polarisation voltage is applied,        along a direction perpendicular to the plane when a polarisation voltage is applied.        
During the application of the polarisation voltage, the magnetisation has a precessional movement around the direction, perpendicular to the plane, of the effective anisotropy. This leads to a magnetisation along an out of plane direction. When the application of the polarisation voltage is stopped, the effective anisotropy and the magnetisation returns to an in plane direction. If a polarisation voltage is applied with a duration corresponding exactly to a half-period (modulo period) of the precession of the magnetisation, then the magnetisation carries out a rotation of an angle π rad compared to the initial magnetisation. Thus the final magnetisation is of opposite direction to the initial magnetisation. The control of the switching of the effective anisotropy thus enables a precessional return of the magnetisation.
However, it is important to note that the change in surface anisotropy ΔKs1(V)=Ks1(0)−Ks1(V) induced by the electric field is relatively low. With an accessible electric field, that is to say below the dielectric breakdown threshold of the insulating layer which is of the order of 109 V/m, the change in surface anisotropy ΔKs1 may typically be nowadays of the order of 10% of the value of the surface anisotropy Ks1(0) when no electric field is applied. In order that the small change in anisotropy causes an important magnetisation variation, it is necessary, by playing on the composition of the materials, the thicknesses of the different materials as well as on the different interfaces, to adjust the effective anisotropy in order that it is situated close to a transition perpendicular to the plane/in the plane. Near to this transition, a 10% variation in the surface anisotropy may in fact be sufficient so that the direction of magnetisation undergoes a rotation, from an “out of plane” direction to an “in the plane” direction and vice versa, thanks to the application of an electric field. Nevertheless, if the system is situated far from such a transition perpendicular to the plane/in the plane for the effective anisotropy, the 10% variation in surface anisotropy induced by the polarisation voltage will not make it possible to change the magnetic orientation and thus will not make it possible to switch the magnetisation. This signifies that such a phenomenon of voltage-controlled anisotropy may be used as a new means of low-consumption writing in a spintronic device uniquely if one is capable of designing the spintronic device considered in such a way that it is situated close to a transition of reorientation of anisotropy between a direction “perpendicular to the plane” and a direction “in the plane”, and for this to apply throughout the entire operating temperature range desired for the device.
As explained previously, the effective anisotropy is adjusted to be close to a transition of reorientation of anisotropy between a direction “perpendicular to the plane” and a direction “in the plane” in order that the electric field is capable of modifying sufficiently the surface anisotropy to switch over the direction of the effective anisotropy and thus be able to lead to a reorientation of the magnetisation. An important problem of the prior art is that this condition of proximity of a transition of reorientation of anisotropy is only generally satisfactory over a very narrow temperature range, because thermal variations in thin magnetic films of the surface anisotropy on the one hand, and of the volume anisotropy on the other hand, are generally different. This is illustrated in FIG. 4. FIG. 4 shows:                the curve “surf(V=0)”, when no polarisation voltage is applied, of the surface anisotropy Ks as a function of temperature;        the curve “surf(V=Vmax)”, when a non-zero polarisation voltage V=Vmax is applied, of the surface anisotropy Ks as a function of temperature;        the curve “vol” of the absolute value of the volume anisotropy multiplied by the thickness of the film ((−½μ0Ms2Kv)*tF) as a function of temperature;        the curve “eff(V=0)”, when no polarisation voltage is applied, of the product of the thickness of the film and the effective anisotropy Keff as a function of temperature;        the curve “eff(V=Vmax)”, when a non-zero polarisation voltage V=Vmax is applied, of the product of the thickness of the film and the effective anisotropy Keff as a function of temperature.        
The effective anisotropy Keff when no polarisation voltage is applied is the sum of the surface anisotropy
                    K                  s          ⁢                                          ⁢          1                    ⁡              (        V        )              +          K              s        ⁢                                  ⁢        2                  t    F  when no polarisation voltage is applied and the volume anisotropy. The effective anisotropy Keff when a non-zero polarisation voltage is applied is the sum of the surface anisotropy
                    K                  s          ⁢                                          ⁢          1                    ⁡              (                  V          =          0                )              +          K              s        ⁢                                  ⁢        2                  t    F  when a non-zero polarisation voltage is applied and the volume anisotropy.
It is generally expected that the surface anisotropy varies more rapidly with temperature than the volume anisotropy, due to the fact that the surface spins have a coordination less than the volume spins and are thus more sensitive to thermal fluctuations.
A reorientation of the magnetisation with an electrical field is possible when, for a zero polarisation voltage V=0, the sign of the effective anisotropy Keff is positive (anisotropy perpendicular to the plane) and when, for a non-zero polarisation voltage V=Vmax, the sign of the effective anisotropy Keff is negative (anisotropy in the plane). FIG. 4 shows the temperature interval ΔT for which the reorientation of the magnetisation with an electrical field is possible. This temperature interval is very narrow.
Yet most electronic devices have to be capable of operating over a wide temperature range: between 0° C. and 70° C. for general public electronics for example, or between −40° C. and 85° C. for industrial electronics. Over such temperature ranges, the condition of proximity of a transition of reorientation of anisotropy is not respected in the prior art. Thus, the electric field controlled devices proposed in the prior art can only operate in a narrow temperature range: they could for example operate correctly at 20° C., but not at 50° C.